A technology concerning turning performance improving control has been developed which improves turning performance of a vehicle to stabilize behavior or a posture of the vehicle by automatically controlling drive force and braking force generated in the vehicle in accordance with a steering operation performed by a driver. As one example, Japanese Patent Laid-Open No. 2011-218953 discloses an invention concerning a drive force control apparatus that controls drive force of driving wheels so that steering characteristics of a vehicle follow target steering characteristic. In the invention described in this Japanese Patent Laid-Open No. 2011-218953, drive force that is increased to relatively improve the steering characteristics of the vehicle is obtained with respect to basic drive force of the driving wheels acquired from a driver's intension. Further, a plurality of upper limit values of the drive force to be increased are calculated based on a plurality of conditions, the drive force to be increased is limited by the smallest upper limit value in the plurality of upper limit values, and a final increment of the drive force is obtained. Specifically, control amount upper limit values are calculated in accordance with longitudinal acceleration of the vehicle, lateral acceleration of the vehicle, and a road surface gradient, respectively. Furthermore, the drive force is controlled to output drive force limited by the smallest one in these upper limit values.
Japanese Patent Laid-Open No. 2011-236810 discloses an invention concerning a vehicle drive force control apparatus that controls drive force to change a stability factor of a vehicle following a target value. In the invention described in Japanese Patent Laid-Open No. 2011-236810, the target value of the stability factor is obtained based on a drive force increase request, and a correction amount of the stability factor is calculated based on a difference between the target value of the stability factor and an actual value of the stability factor. Then, the drive force is increased to reduce a difference between the target value and the actual value of the stability factor.
In turn, Japanese Patent Laid-Open No. 7-101272 discloses an invention intended to estimate a vehicle driving operation state of a driver that is hard to directly represent in physical quantity such as vehicle speed and provide vehicle driving characteristics matched with a result of this estimation. In the invention described in Japanese Patent Laid-Open No. 7-101272, a neural network that receives average values and variances of respective parameters provided by frequency analysis of vehicle driving parameters, e.g., vehicle speed as well as parameters representing road traffic situations performs nonlinear transformation with respect to a weighted total sum of the average values and the variances of the vehicle driving parameters and the parameters of the road traffic situations to obtain an output parameter representing the vehicle driving operation state. Furthermore, the vehicle driving characteristics are changed in accordance with this output parameter.
In the respective inventions described in Japanese Patent Laid-Opens No. 2011-218953 and No. 2011-236810, so-called turning performance improving control is carried out. In the invention described in Japanese Patent Application Laid-Open No. 2011-236810 of these inventions, drive force of a vehicle is controlled so that a stability factor of the vehicle follows a target stability factor. Thus, vehicle behavior during turning traveling can be stabilized to improve turning performance of the vehicle. In such conventional turning performance improving control as described in Japanese Patent Laid-Open No. 2011-236810, drive force or braking force of a vehicle is automatically controlled aside from an increase or a decrease in drive force and braking force caused by an accelerator operation or a brake operation of a driver. Thus, there is a problem that the drive force (or the braking force) fluctuates without the driver's intention and the driver feels this fluctuation as uncomfortableness or shocks.
As regards such a problem, like the invention described in Japanese Patent Laid-Open No. 2011-218953, providing a limit value to a control amount of the drive force at the time of executing the turning performance improving control is considered. For example, as shown in FIG. 7, a drive force limit value tqgd is usually set to the drive force output to follow a required drive force. In the example shown in FIG. 7, when steering begins and a vehicle makes a turn, the turning performance improving control is executed, which results in a reduction in actual drive force of the vehicle. Moreover, the reduction in this actual drive force is limited with the limit of the drive force limit value tqgd.
A limit value that limits a control amount of the drive force in the turning performance improving control can be preset by, e.g., drive tests or simulations as a limit value that prevents a driver from feeling uncomfortableness or shocks. Additionally, applying the concept of Weber's law “a differential threshold of stimuli (ΔX) varies in proportion to intensity of reference stimuli (X) (ΔX/X=constant)”, the limit value of a control amount can be set as a variable value. That is, determining the limit value of a control amount in accordance with a level of the control amount can be also considered. As described above, setting a limit to the control amount of the drive force at the time of performing the turning performance improving control enables preventing or suppressing a driver from feeling uncomfortableness or shocks caused due to a change in drive force during turning traveling.
However, when the control amount of the drive force is restricted at the time of performing the turning performance improving control, a factor different from that described above may possibly bring uncomfortableness to a driver. That is, changing the drive force during turning traveling can suppress the uncomfortableness the driver feels. However, when the control amount of the drive force is restricted, control effect of the turning performing improving control is lowered. Further, when the control effect is lowered, the control effect of the turning performance improving control expected by the driver is not provided, which may possibly lead to different comfortableness to the driver.
In particular, when a so-called steering angle increasing operation is carried out to again increase a steering angle once fixed at a predetermined angle at the time of turning traveling, a difference between the expected control effect and actually provided control effect, i.e., insufficiency of the actually provided control effect becomes prominent. Consequently the driver is prone to the uncomfortableness or shocks. For example, as shown in FIG. 8, when the steering angle increasing operation is performed, actual drive force is limited by the drive force limit value tqgd, whereas required drive force required to provide appropriate control effect increases (in an example shown in this FIG. 8, it increases toward a negative side or a braking side). Consequently, the required drive force considerably deviates from the actual drive force, and the driver may feel different uncomfortableness that prevents the expected control effect of the turning performance improving control from being provided depending on circumstances.
It is to be noted that not only the control amount of the drive force of the vehicle is restricted to reduce the uncomfortableness or shocks at the time of performing the turning performance improving control as described above but also the control amount may be restricted in association with management of operation states of an engine or states of a battery. Even in such a case, like the limitation for reduction of the uncomfortableness or the shocks described above, the control amount of the drive force is limited, and hence the control effect of the turning performance improving control is lowered.
Thus, the conventional control systems have to be improved to carry out the turning performance improving control for stabilizing vehicle behavior during turning for the purpose of enhancing control effect while reducing uncomfortableness or shocks to a driver.